That is a very good question, indeed. I imagine that it would be hard for the system to tell whether the puzzle was completed genuinely, or whether they went back a few lessons and started clicking "Run" on the partner's completed puzzles to fill in their progress.
I would suppose that this would make an important teaching moment. Helping the students see the difference between communicating about their code to rewrite on a partner's computer, versus just auto-filling the solutions.
That said, I don't know what the log-out policy is, so if their accounts were still connected and the student who was ahead went home to show her parents, then it all could have been very unintentional.
We'll take a deeper look. In the meantime, please let us know if you discover anything.